We had precisely this issue with one of my DS's mates at the same age. Very annoying. I spoke to the parents and asked them to not allow their own DS to call my mobile except in an emergency and told them flat out that 10 calls a day was driving me nuts, not to mention very disruptive when I was at work. The next time the child rang I told him that my DS couldn't talk and that I did not want him to call my mobile again. The problem ceased. It might be a bit more difficult if the parents of the child who is calling are not on the same page as you! I would still speak to them and ask them to not allow their child to call your mobile. Tell them that your own child is not allowed to use your mobile phone and that you are finding the numerous incoming calls disruptive and annoying.